1. Field of Invention
The invention concerns capped machine nuts comprising a polygonal-sided cold-formed, forged or extruded metal nut body, and a cup-shaped sheet metal cap enclosing one end face and the side walls of the body. The invention is more particularly concerned with capped wheel nuts used in detachably securing automobile wheels to their hubs.
2. Description of the Prior Art
It has been standard practice in the automotive industry for many years to use hub caps or wheel covers to enclose the circle of lugs and lug nuts used to detachably secure the wheels to the axle or spindle hubs. Such hub caps or wheel covers serve to protect the fastening means from exposure to road dirt, rust or other corrosion, etc., which impede removal and replacement of a wheel. The covers have also served as means for enhancing the appearance of the wheel. However the covers are expensive, add weight to the car and often become lost due to failure, under the extreme stresses imposed by high speed and rapid maneuvering of the vehicle, of the means employed to attach the covers to the wheels. Accordingly many car manufacturers have redesigned their wheel structures to eliminate the use of wheel covers, but this of course leads back to exposure of the studs or lugs and wheel nuts; i.e., to the very conditions that brought on resort to wheel covers in the first place. The use of capped nuts has been proposed as a solution. These provide individual protection of the exposed portions of the mating threads of the lugs and nuts, more particulary at the axial outer end of the lugs. The lug nuts themselves must be made of machinable grades of steel, in order to permit satisfactory fabrication of the threads and to provide the required strength, yet be as low cost as possible. Such steel, however does not lend itself readily to good nickel/chrome plating, which is desired for the sake of better appearance since the nuts will be fully visible when installed on a car. Therefore machinable steel has been used for the nut body, and this body is then combined with a separate sheet metal cap of non-corrosive stainless metal, again usually steel. The cap is designed to cover the axial outer end of the nut body and as much of its side walls as is necessary for appearance design considerations. The cap must of course be secured to the nut body adequately to withstand the very heavy torque loads produced by the usual impact wrench forces involved when the car wheels are removed and replaced because of flats, tire wear, rotation or snow tire replacement, for example. Such impact wrench forces are very substantial and inevitably produce slight deformation or other distortion of the nut body, as well as peening and stretching of the metal in the cap. Thus, even though a tight, close fit may be initially obtained between the nut body and its cap, this distortion, stretching, etc., tends to loosen the caps and nut bodies, and annoying rattles result.
The prior art has experienced this and other difficulties unique to automotive wheel nuts, and several solutions have been proposed. Probably the currently most significant wheel nut proposals are disclosed in the U.S. Pat. Nos. to Chaivre 3,364,806, dated Jan. 23, 1968, and Rommano 4,015,503, dated Apr. 5, 1977. Capped wheel nuts of both types disclosed in these patents have been produced commercially and used by major automobile companies. Both types are subject to certain difficulties or disadvantages. For example, capped wheel nuts of the kind disclosed in the earlier patent mentioned above, when manufactured on a mass production scale to meet demand and be economically feasible, are subject to loosening and rattling of the cap after only a very few on-off cycles using a conventional impact wrench. Industry requirements usually specify that the nuts be capable of withstanding at least 10, preferably 12, on-off cycles using commercial impact wrenches. Attempt has been made to spot-weld the cap to the nut body to supplement the cap-attaching method shown in the patent. This welding operation is not easy to do in automated mass production, adds another manufacturing operation, is expensive and tends to impair the appearance of the finished nut even when special buffing or polishing is employed. Improvements in cap retentive capability is claimed for the nut structure shown in the second patent mentioned above, but that design requires the nut body to be accurately machined, as on an automatic screw machine, to provide a characterizing low-angled conical surface and reversely conical shelf as the surface against which the lip of the cap is formed by peripherally continuous swaging of its lip to effect a grip adequate to retain the cap on the nut body. Since an accurate screw machining operation is involved, it is not feasible to use low-carbon, lower cost steels because poor chip formation of such steels is a major problem in screw machining procedures. Thus more expensive steels for the nut bodies is required. Other problems of manufacture and use may also be encountered.